Very numerous: Fermanagh & Ulster generally, Dublin, South East, Mid-West. Ir. Mac Braoin, a sept of Kilkenny and Ó Braoin, septs of the Midlands & Roscommon.This name occurs as O'Brien and Bruen in some areas. Braon, an early personal name. IF.

Brian:

rare: Down etc. Ir. Ó Briain, but in this case, Scottish. The personal name Brian was popular amongst the Normans who got it from their Breton allies and so it found its way to Scotland independently of its Irish currency. SS & GPN.

Brien:

fairly numerous: Dublin etc. See Bryan and O'Brien.

Bryan:

numerous: Ulster, Dublin, Cork, South East. Anglo-Norman associated with Kilkenny. The prevalence in Fermanagh suggests Mac Braoin, a branch of Mac Manus, but the large numbers in the Belfast area are likely to be of Scottish origin.

Very numerous: all areas, most in Munster and Leinster. Ir. Ó Briain. Leading sept of Thomond (Clare-Limerick), owing prominence to Brian Boru, High-King 1002-1014 A.D. The first name Brian is an old Celtic one, meaning perhaps "noble". GPN.